Because of its inherent robustness against multipath fading in wireless communications, Orthogonal Frequency Division Multiplexing (OFDM) is one of the most popular technologies used in modern high data-rate wireless communication systems. The OFDM is a modulation method in which a wideband channel is divided into a set of K parallel narrowband subcarriers with equal bandwidth. Each subcarrier is modulated by a low-rate data stream, whose data rate is 1/K of the entire high-rate stream. When the subcarrier bandwidth is much smaller than the coherent bandwidth of the wireless channel, the wideband channel with frequency selective fading is turned into K narrowband channels with frequency-flat fading. Furthermore, inserting a guard interval, or cyclic prefix (CP) between adjacent OFDM symbols removes the inter-symbol-interference (ISI) and inter-carrier-interference (ICI) caused by the multipath propagation effects. The wideband channel is thereby converted into a set of parallel flat-fading narrowband sub-channels, each sub-channel characterized only by a single complex multiplicative gain.
For a coherent modulated system, this channel gain needs to be estimated at the receiver for each of the subcarriers in order to perform channel equalization and data detection. Estimation of the subcarrier channel gains requires both the transmission of pilot symbols and additional signal processing complexity at the receiver, which is undesirable in some applications. The channel estimation requirements become less stringent when a differential modulation format such as the differential phase shift keying (DPSK) is used. Accordingly, a combination of DPSK modulation with OFDM enables to combine a relatively good robustness against multipath fading and a simple receiver implementation. This combination is used in the Digital Audio Broadcasting (DAB) and Digital Multimedia Broadcasting (DMB) standards, also known collectively as Eureka 147 DAB/DMB standard, which are examples of OFDM systems utilizing differential phase modulation, in particular the, π/4-DQPSK (differential quadrature phase shift keying), in each subcarrier. Eureka 147 DAB has a number of country specific transmission modes, and spectra have been typically allocated for it in Band III (174-240 MHz) and L band (1452-1492 MHz). For worldwide operation a DAB receiver must support the following 4 modes: Mode I for Band III, Earth; Mode II for L-Band, Earth and satellite; Mode III for frequencies below 3 GHz, Earth and satellite; Mode IV for L-Band, Earth and satellite.
Using values for the most commonly used transmission mode on DAB, Transmission Mode I (TM I), the OFDM modulation consists of 1,536 subcarriers that are transmitted in parallel. The useful part of the OFDM symbol period is 1 millisecond, which results in the OFDM subcarriers each having a bandwidth of 1 kHz, and the overall OFDM channel bandwidth is 1,537 MHz. The CP duration for TM I is 246 microseconds, so that the overall OFDM symbol duration is 1.246 milliseconds. The CP duration also determines the maximum separation between transmitters that are part of the same single-frequency network (SFN), which is approximately 74 km for TM I.
Despite its advantage against multipath fading, a conventional OFDM system suffers from two drawbacks in fast-time varying channels. Due to its much longer symbol duration, the subcarrier response between two adjacent symbols is less correlated, which results in significant performance degradation for differential detection when the channel changes faster than the symbols. The second drawback comes from the loss of orthogonality of OFDM subcarriers caused by the Doppler frequency shifts, which appear in mobile reception. Doppler spread generates inter-carrier-interference (ICI), wherein the received signal in one subcarrier contains a certain amount of signal power leaked from adjacent subcarriers. Unlike the thermal noise, which is conventionally denoted as the AWGN noise (additive white Gaussian noise) hereinafter, the effect of ICI cannot be reduced by increasing transmission signal power, since the ICI power increases when the signal power is increased. Therefore, ICI usually results in an error floor in the system bit error rate (BER) performance.
A DAB network can work in one of the four transmission modes, which are characterized by different system parameters, including the number of subcarriers, the subcarrier spacing, the OFDM symbol duration as well as the CP length, as shown in Table 1.
TABLE 1System parameters of four DAB modesModeParametersIIIIIIIVSNo. symbols/frame767615376KNo.1536384192768NFFT size (points)20485122561024TsTotal symbol duration~1246~312~156~623TuUseful symbol1000250125500TGGuard interval~246~62~31~1231/TuSubcarrier spacing1482(kHz)TFFrame duration (ms)96242448
In a single-frequency network (SFN) configuration, the length of the CP determines the spacing between adjacent DAB transmitters. Therefore, the longer the CP is, the larger the separation between DAB transmitters can be and the fewer the number of transmitters required to cover an entire SFN network. On the other hand, the performance of DAB receivers depend on the normalized Doppler spread, fdTs, defined as the product of the single-sided Doppler spread fd and the OFDM symbol duration, Ts The Doppler spread is proportional to both the RF carrier frequency and the vehicle speed. For large values of fd, obtained for instance with DAB transmission at L-band and with car speeds greater than 100 km/h, a DAB system operated in mode III (shortest Ts) would perform the best while mode I would perform the worst (longest Ts).
Fast fading channels remain a challenge to conventional OFDM systems using differential modulation. For instance, it was shown that the receiver speed is limited to 95 km/h for DAB transmission with mode IV at L-band, for a channel which has the so-called typical urban (TU) multipath power delay profile. This suggests that a satisfactory DAB service is not available for most vehicle receivers running on freeways. Therefore, better detection techniques need to be developed in order to extend the speed limit of reliable operation of wireless OFDM receivers with differential detection such as the DAB/DMB receivers.
It is therefore an object of the present invention to provide a receiver for receiving a differentially modulated multi-carrier signal with an improved performance in fast fading channels.